KR20080105088A - Process for the preparation of an alkanediol and a dialkyl carbonate - Google Patents

Abstract

An alkanediol and a dialkyl carbonate are prepared in a process comprising: (a) contacting an alkylene carbonate with an alkanol feedstock under transesterification conditions in a reactive distillation zone to obtain a top stream comprising dialkyl carbonate and the alkanol and a bottom stream comprising the alkanediol; (b) separating the top stream comprising dialkyl carbonate and the alkanol into an alkanol-rich stream and a dialkyl carbonate-rich stream; (c) recovering the dialkyl carbonate, optionally after further purification, from the dialkyl carbonate-rich stream; and (d) recycling at least part of the alkanol-rich stream to the reactive distillation zone as part of the alkanol feedstock, wherein the alkanol-rich stream is split in at least two portions, and at least one portion is condensed and freed from compounds with a lower boiling point than the alkanol. The process is especially suitable for the production of propylene glycol and dimethyl carbonate from propylene carbonate and methanol. ® KIPO & WIPO 2009

Description

PROCESS FOR THE PREPARATION OF AN ALKANEDIOL AND A DIALKYL CARBONATE

The present invention relates to a process for the preparation of alkanediol and dialkyl carbonates. More specifically, the present invention relates to a process for the preparation of said compounds from alkylene carbonates and alkanols.

The method is known, for example, from US-A 5359118. This document discloses how the transesterification of C ₁ -C ₄ alkanols with alkylene carbonates produces di (C ₁ -C ₄ alkyl) carbonates. Here, the alkylene carbonate and alkanol feedstocks are reacted in countercurrent in the tower. Alkylene carbonate enters the upper part of the tower and flows from above. An alkanol feedstock comprising pure alkanols, and a stream comprising alkanols and also dialkyl carbonates enter the tower at the bottom. The alkanol moves upwards and reacts countercurrently with the alkylene carbonate to obtain dialkyl carbonate and unreacted alkanol as the top effluent, and any incorporated alkanol and alkanediol as the bottom effluent. The top effluent obtains an alkane rich stream comprising alkanol and small amounts of dialkyl carbonate by extractive distillation. This stream is fed to the tower as part of the alkanol feedstock.

The method discloses the production of high boiling by-products such as polyglycol. However, known methods do not solve the problem of accumulation of low boiling by-products. Such by-products can be carbon dioxide, which can be produced, for example, due to hydrolysis of alkylene carbonates with small amounts of water that may be present in alkanols or any other starting material. Other byproducts that may be produced include acetaldehyde, propionaldehyde, and acetone.

It has now been found that accumulation of low boiling by-products can be prevented by generating a bleed stream of alkanols. Accordingly, the present invention provides a process for preparing alkanediol and dialkyl carbonate,

(a) contacting an alkylene carbonate with an alkanol feedstock under transesterification conditions in a reaction distillation zone to produce a top stream comprising dialkyl carbonate and alkanol and a bottom stream comprising alkaninediol;

(b) separating the top stream comprising dialkyl carbonate and alkanol into an alkanol rich stream and a dialkyl carbonate rich stream;

(c) recovering the dialkyl carbonate from the dialkyl carbonate rich stream; And

(d) recycling at least a portion of the alkanol-rich stream to the reaction distillation zone as part of the alkanol feedstock,

The alkanol-rich stream is here separated into at least two portions, and at least one portion is condensed to provide a process that is free of compounds having a lower boiling point than alkanol.

According to the invention an alkanol rich stream is separated from the top stream of the extractive distillation. At least one portion is condensed. This condensation can be achieved by many methods. An advantageously used method for condensing at least part of the vapor is to pass this stream through a liquid or air cooler. From the condensed liquid, the low boiling compounds in the vapor phase are separated and removed in this process. In this way accumulation of low boiling compounds is prevented.

The process of the present invention involves transesterification of alkylene carbonates with alkanols. Such transesterification reactions are known and are for example clearly shown in US Pat. No. 5,359,118. The starting material of the transesterification reaction is preferably selected from C ₂ -C ₆ alkylene carbonates and C ₁ -C ₄ alkanols. More preferably the starting materials are ethylene carbonate or propylene carbonate and methanol, ethanol or isopropanol. Most preferred alkanols are methanol and ethanol.

The transesterification step is advantageously carried out in a column in which the alkylene carbonate is introduced from the top to countercurrently contact with the upwardly moving alkanol and allow the alkylene carbonate to flow off. The products of this reaction are dialkyl carbonates and alkanediols. Dialkyl carbonate is recovered in the top stream at the top of the tower. Alkanediol is recovered in the bottom stream.

The transesterification reaction suitably proceeds in the presence of a catalyst. Suitable catalysts are described in US-A 5359118 and include hydrides, oxides, hydroxides, alcoholates, amides or salts of alkali metals such as lithium, sodium, potassium, rubidium and cesium. Preferred catalysts are alcoholates or hydroxides of potassium or sodium. It is advantageous to use alcoholates of alkanols which are used as feedstocks. Such alcoholates may still be added or may be produced in situ.

Another suitable catalyst is an alkali metal salt such as acetate, propionate, butyrate, or carbonate. Further suitable catalysts are described in US-A 5359118 and the documents cited therein, such as EP-A 274953, US-A 3803201, EP-A 1082, and EP-A 180387.

The conditions of the transesterification reaction are known in the art and suitably include a temperature of 40 to 200 ° C. and a pressure of 50 to 400 kPa. Preferably, the pressure is close to atmospheric pressure. The temperature depends on the alkanol feedstock and the pressure used. The temperature is maintained to be close to or above the boiling point of the alkanol, for example up to a temperature of 5 ° C. above the boiling point. For methanol and atmospheric pressure, the temperature may be at least 65 ° C, such as 65 to 70 ° C.

The transesterification reaction is advantageously carried out in a tower equipped with internals such as a distillation column. Therefore, it may include sweets, bubble trays, or Raschig rings with bubble caps. Those skilled in the art will appreciate that various packings and stage configurations will be possible. Alkylene carbonate can enter and flow down the top of this column. Alkylene carbonates generally have a higher boiling point than alkanols. For ethylene and propylene carbonates the atmospheric boiling point is at least 240 ° C. The alkylene carbonate comes in contact with the alkanol flowing upwards down the stage or ring. When the transesterification catalyst is homogeneous, such as alkali metal alcoholate, it is introduced at the top of the tower. Alkanol feedstock comes from a lower point. This feedstock may be completely vaporous. However, the feedstock may also enter the tower in part liquid state. It is believed that the liquid phase has a beneficial effect on the overall transesterification reaction by securing a higher concentration of alkanol at the bottom of the column. It is distributed throughout the width of the tower through the inlets and the interior of the tower. The proportion of the vapor and liquid portions of the alkanol feedstock can vary within wide ranges. The vapor / liquid weight ratio is suitably from 1: 1 to 10: 1 weight / weight.

Those skilled in the art will appreciate that the transesterification reaction is an equilibrium reaction. Therefore, it is appropriate to use excess alkanol. The molar ratio of alkanol to alkylene carbonate is suitably 5: 1 to 25: 1, preferably 6: 1 to 15: 1, more preferably 7: 1 to 9: 1. The amount of catalyst can obviously be much lower than this. Suitable amounts include from 0.1 to 5.0% by weight, preferably from 0.2 to 2% by weight, based on alkylene carbonate.

Reactive distillation produces a top stream comprising dialkyl carbonate and any excess unreacted alkanol, and a bottom stream comprising alkanedinol and a catalyst. Because of the slight water that may be included in the alkanols, some hydrolysis of the alkylene carbonates that produce alkanediol and carbon dioxide may occur. Other low boiling by-products or impurities are aldehydes, ketones and alkylene oxides.

The bottoms stream suitably undergoes separation of alkanediol. Also, the bottoms stream is suitably divided in a fractional distillation column into a catalyst rich stream and a stream comprising alkanediol and optionally some alkanols. After purification, the alkanedinol is recovered to the final product, such as by further distillation. The catalyst rich stream is suitably recycled to the reaction distillation zone. Also any alkanols separated from the bottoms stream can be recycled.

The top stream is sequentially separated into alkanol rich streams and dialkyl carbonate rich streams. This may suitably be carried out by distillation. However, as pointed out in US-A 5359118, many alkanols and their corresponding dialkyl carbonates form an azeotrope. Therefore, distillation will not be enough to achieve satisfactory separation. It is therefore preferred to use extractants to facilitate the separation of dialkyl carbonates and alkanols. The extractant may be selected from a number of compounds, especially alcohols such as phenols or anisoles. However, preference is given to using alkylene carbonates as extractant. It is most advantageous to obtain a separation in the presence of alkylene carbonate being used as starting material for the final alkanedinol.

Extractive distillation is preferably carried out in two towers. Separation in the first column is obtained between the alkanol and dialkyl carbonate / alkylene carbonate mixtures. In the second tower, separation of dialkyl carbonates and alkylene carbonates is obtained. The alkylene carbonate may be recycled to the first tower suitably for regeneration use as extractant. The proportion of alkylene carbonates and alkanols and alkylene carbonates and dialkyl carbonates can vary over a wide range. Suitable ranges include 0.2 to 2 mol, preferably 0.4 to 1.0 mol, of alkylene carbonates per total mole of alkanols and dialkyl carbonates.

Distillation conditions for the separation can be selected within a wide range and will be understood by those skilled in the art. The pressure suitably ranges from 5 to 400 kPa and the temperature ranges from 40 to 200 ° C. In view of the stability of the alkylene carbonates the temperature is advantageous below 180 ° C. while the lower temperature is determined by the boiling point of the alkanol. When two distillation columns are used, the separation of the alkanol and dialkyl carbonate / alkylene carbonate mixtures is carried out at higher pressures, such as 60 to 120 kPa, and the second separation between dialkyl carbonates and alkylene carbonates is at lower pressures. For example, it is preferably performed at 5 to 50 kPa. This will allow a temperature low enough to maintain satisfactory stability of the alkylene carbonate and allow efficient separation of the carbonate compounds. The dialkyl carbonate obtained is optionally recovered as product after further purification. Such additional purification will include additional distillation steps or ion exchange steps, as described in US-A 5455368.

The alkanol rich stream obtained is free of low boiling compounds. In one aspect the top stream is separated into two or more portions. At least one portion is cooled to condense alkanols. Any low boiling compounds in the vapor phase are removed. The condensed alkanol is then recycled to the reaction distillation zone. Although some low boiling compounds may be incorporated into the liquid phase, the present method provides satisfactory removal of such compounds. In another embodiment the liquid alkanol is recycled to step (b) after condensation. Step (b) involves the liquid portion of the alkanol being sent to the distillation column at reflux. Combinations of both recycles are also possible.

US-A 5359118 mentions that an alkanol-rich stream of vapor phase from this distillation is recycled to the reaction distillation zone. The document further describes that the recycled alkanol enters the reaction distillation zone at the upper point rather than the inflow of pure complement alkanol. According to this document, the recycle stream comprises another compound, in particular dialkyl carbonate.

In the present invention, an alkanol-rich stream is separated into at least two parts, typically two are sufficient. At least one portion is then compressed and recycled to the reaction distillation zone in a vaporous recycle stream. At least another portion is condensed to remove the low boiling compounds.

The weight ratios of the separated portions of the liquid and vapor phases may be chosen by those skilled in the art to achieve the optimum effect. It is advantageous to separate the alkanol rich stream so that the weight ratio of the condensed portion and the vapor phase portion can be 0.1: 1 to 1: 1. This will allow impurities to be removed efficiently.

As stated above, the alkanol rich stream is suitably recycled to the reaction distillation zone. Therefore, alkanol feedstocks advantageously comprise complementary pure alkanols, and liquid and vapor phase recycle streams. The recycle stream can be mixed with complementary pure alkanols and subsequently introduced into the reaction distillation zone as an alkanol feedstock. It is also possible to combine the liquid or vapor recycle stream with complementary pure alkanols before entering the distillation zone. However, it is most preferable to introduce the supplemental alkanol into the reaction distillation zone at the bottom rather than introducing the liquid and vapor recycle streams. This results in the advantages described in US-A 5359118.

The method of the present invention can be used for a variety of feedstocks. The process is excellently suitable for the production of ethylene glycol, propylene glycol, dimethyl carbonate and / or diethyl carbonate. The process is most advantageously used for the production of propylene glycol (1,2-propane diol) and dimethyl carbonate from propylene carbonate and methanol.

1 shows an embodiment of the invention in which a portion of the alkanol-rich stream free of impurities is recycled to the separation of alkanol and dialkyl carbonates in the extractive distillation zone.

FIG. 2 shows another embodiment of the invention in which a portion of the alkanol-rich stream free of impurities is recycled to the reaction distillation zone.

1 shows reactive distillation zone 1 and two extractive distillation zones 2 and 3. The method will now be described taking propylene carbonate and methanol as an example. It is understood that one skilled in the art can replace the example with any other suitable alkylene carbonate and alkanol.

Through line 4 propylene carbonate enters the top of the reaction distillation zone 1. The transesterification catalyst is also sent to the top of zone 1 via line 5. Methanol entering the bottom of Zone 1 through lines 6 and 7a moves upwards and is activated by a transesterification catalyst to react with propylene carbonate to produce propylene glycol product and dimethyl carbonate. Propylene glycol is recovered from the bottom of distillation zone 1 via line 8. The bottom product of line 8 also includes a catalyst. Therefore, the product is separated into a portion containing the catalyst and a product portion in separation apparatus 9. Separation may be accomplished by distillation. The catalyst is recycled to zone 1 via line 5 and propylene glycol is recovered via line 10 after optional further purification (not shown).

It is appropriate to use methanol in stoichiometric excess. Therefore, the mixture of methanol and dimethyl carbonate passes through the top of Zone 1 via line 11 to the first extractive distillation Zone 2. Through line 12 propylene carbonate enters the extractive distillation zone 2. The extractant, propylene carbonate, enters the extraction zone at a higher stage than the methanol and dimethyl carbonate mixture. Extractive distillation separates the methanol-rich product, which passes through the top of Zone 2 via line 7. The product is separated into two parts. One portion is recycled to the reaction distillation zone 1 via line 7a with recycled methanol. Recycle methanol is advantageously introduced into reaction zone 1 at a higher stage than (complement) methanol supplied via line 6. The other part is sent to separation device 17 via line 7b. In this separation device the stream is separated into a stream containing compounds with a lower boiling point than methanol, such as gaseous impurities such as carbon dioxide, and a liquid stream which is predominantly methanol. Airflow exits the process via line 18. Liquid methanol is recycled to extractive distillation zone 2 via line 7c.

From the bottom of the extractive distillation a mixture of mainly dimethyl carbonate and propylene carbonate is obtained. This mixture is sent via line 13 to the second extraction reaction zone 3. In this distillation, dimethyl carbonate is recovered via line 14 at the top, while propylene carbonate is recovered at the bottom. Through line 15 the propylene carbonate is separated into a stream entering the reaction distillation zone 1 via line 4 and a second stream recycled to the extraction distillation zone 2 via lines 16 and 12.

In FIG. 2, the same reference numerals apply to the same apparatus as in FIG. The mixture of dimethyl carbonate and methanol in the manner described for the process according to FIG. 1 leaves the reaction distillation zone 1 via line 11 and undergoes extractive distillation in extractive distillation zone 2. The methanol-rich product separated from this mixture exits the top of the extractive distillation zone 2 via line 7. This product is separated into at least two parts. One portion can be used as distillation reflux (denoted via line 7d). Another portion returns directly to the reaction distillation zone 2 via line 7a. The third part enters cooling unit 19 via line 7e, and the product having a lower boiling point than methanol is separated. This light product is discharged via line 18. The cooled methanol is recycled to the reaction distillation zone 1 via line 7f. Recycling can be accomplished by combining lines 7f and 7a (as shown) or by connecting lines 7f alone to the reaction distillation zone.

Example 1

120 g / h of propylene carbonate and 1.5 g / h sodium methanolate in a mixture of 15 g / h propylene glycol and methanol are continuously introduced to the top of the reaction distillation column to show the production of by-products in a continuous process. At the bottom of the column is 340 g / h methanol. The reaction distillation column is operated at pressures varying from 135 to 105 kPa and at temperatures ranging from 63 to 119 ° C. 365 g / h of methanol and dimethyl carbonate mixture is continuously discharged from the distillation column from the top of the tower. The weight ratio of the effluent stream was about 70 wt% methanol and about 30 wt% dimethyl carbonate. The amount of by-products is as shown below.

by-product Volume, ppm Acetaldehyde 20 Propylene oxide 450 Propionaldehyde 20 Acetone <20

This mixture was introduced into an extractive distillation column fed with 1400 g / h of propylene carbonate. A methanol flow of 255 g / h was obtained at the top of the extraction distillation column operating at a temperature of about 180 ° C. and a pressure of 102 kPa. The methanol stream had a purity of 99% by weight or more, but included the impurities below.

by-product Volume, ppm Acetaldehyde 70 Propylene oxide 1000 Propionaldehyde 40 Acetone <20 Dimethyl carbonate 400

This example clearly shows that small amounts of byproducts are produced in a continuous process.

Example 2

In the process described in FIG. 1, 7 t / h (tons per hour) of propylene carbonate enters the top of the reaction distillation zone. Sodium methanolate, a transesterification catalyst in a 0.5 t / h propylene glycol / methanol mixture, is also introduced to the top of the zone in an amount of 0.05 t / h. 20 t / h of methanol entering the bottom of the distillation zone moves upward and reacts with propylene carbonate to produce propylene glycol product and dimethyl carbonate. Propylene glycol is recovered in an amount of 5 t / h.

The methanol and dimethyl carbonate mixture exits the top of the reaction distillation zone and comprises 13 t / h methanol, 5.2 t / h dimethyl carbonate, 22 kg / h propylene oxide and about 55 kg / h other volatile compounds. The mixture is separated by extractive distillation. The separation yields a methanol rich stream comprising 13 t / h methanol, trace amounts of dimethyl carbonate, and all of the other by-products mentioned above.

Half of this flow was condensed at 59.3 ° C. and 92 kPa, yielding 2 kg / h of propylene oxide, about 25 kg / h of other volatile compounds and 175 kg / h of methanol. The steam was vented. The liquid stream comprising the remaining methanol and residual byproducts is recycled to the reaction distillation zone as part of the methanol feed.

In this way it is clear that the accumulation of by-products in the process is being controlled.

Claims (6)

As a process for preparing alkanediol and dialkyl carbonate, (a) contacting an alkylene carbonate with an alkanol feedstock under transesterification conditions in a reaction distillation zone to produce a top stream comprising dialkyl carbonate and alkanol and a bottom stream comprising alkaninediol; (b) separating the top stream comprising dialkyl carbonate and alkanol into an alkanol rich stream and a dialkyl carbonate rich stream; (c) recovering the dialkyl carbonate from the dialkyl carbonate rich stream, optionally after further purification; And (d) recycling at least a portion of the alkanol-rich stream to the reaction distillation zone as part of an alkanol feedstock, Wherein the alkanol-rich stream is divided into at least two portions and at least one portion is condensed so that there is no lower boiling compound than alkanol. The process of claim 1 wherein the separation of step (b) is carried out in the presence of an alkylene carbonate. The process of claim 1 or 2, wherein the portion of the compound having a lower boiling point than the alkanol is recycled to the reaction distillation zone. The process according to claim 1 or 2, wherein the part without the lower boiling point compound than the alkanol is recycled to the separation of step (b). The process according to claim 1, wherein the weight ratio between the condensed part and the vapor part is from 0.1: 1 to 1: 1. The method of claim 1, wherein the alkylene carbonate is propylene carbonate and the alkanol is methanol.

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